At Christmas, thousands of greetings cards feature the iconic winter plumage of the聽. But not all the robins you might find in your backyard are permanent natives to your country. In the UK, for example, some will have migrated from聽, and like millions of other animals all over the world, will return back to their breeding grounds next spring. This聽聽of birds, mammals, fish and insects is a key part of the Earth鈥檚 biodiversity.
聽is a challenging strategy. For small animals like songbirds to be able to return from Africa or southern Europe to areas where they can successfully breed, they need to be able to repeatedly navigate to precisely the same place. How they do this is a question that has vexed scientists for over 60 years.
叠耻迟听聽is shedding light on how tiny animals 鈥 with correspondingly聽聽鈥 can cross mountains, oceans and deserts without getting lost.
It seems that birds use聽聽available in their environment, like the stars and the Earth鈥檚 magnetic field, to provide them with the equivalent of a map and a compass. But how they combine these cues is only now starting to become clear.
Our research group鈥檚聽聽focuses on how聽, a small Eurasian songbird, navigate. First, we disrupted the birds鈥 ability to sense the Earth鈥檚聽聽by attaching a small magnet to their forehead.
We found that when we also obscured their view of the stars, the birds were unable to find the right direction to migrate in. Once their view of the stars was returned, however, they were able to find their way again.
Like a careful engineer, the evolutionary process of聽聽has built in a fail-safe to birds鈥 navigation systems, making sure there are backup orientation devices available for when the skies are cloudy.
Magnetic fields
聽by our research group has shown how these same reed warblers can work out exactly where they are when migrating, as well as how they deal with being blown off course or needing to detour around barriers like the聽.
We put reed warblers in an artificial magnetic field that matched the natural magnetic field of a place far northwest of the birds鈥 migration route. We then tested their sense of direction in an聽聽鈥 a small funnel 30cm in diameter that allows us to measure the direction a bird wants to take off in by analysing where it hops inside the cage.
We found that when placed in this artificial field, birds changed their orientation from southeast to southwest, suggesting they had recognised the magnetic field signature as foreign and were trying to get back to their route.
We call this technique 鈥溾, as the bird itself never actually leaves the site where it鈥檚 captured for testing. It鈥檚 become a new tool for understanding how animals sense and use the Earth鈥檚 magnetic field for navigation.
What was even more remarkable was that the artificial magnetic field we created is not one that the birds would have previously encountered on their migrations. That means they weren鈥檛 reacting to magnetic field cues that they had learned. Instead, the birds had used their instinctive awareness of how the Earth鈥檚聽聽changes with distance to work out that they were northwest of their route. Not bad for those tiny bird brains
Clearly, both the starfield and the magnetic field are important cues for birds to migrate. But human activity has the potential to聽听迟丑别蝉别.听聽at night from cities reduces visibility of the stars and moon. In other animals, such as聽听补苍诲听, this has been shown to negatively affect navigation abilities.
What鈥檚 more, artificial electromagnetic signals 鈥 such as those coming from radio towers or even from electric currents that power everyday devices such as kettles 鈥 can also聽聽birds鈥 ability to detect the natural magnetic field. We may be giving birds a double dose of pollution that even their fail-safe systems can鈥檛 overcome.
We don鈥檛 fully understand how these pollutants affect migratory birds yet, but as we come to understand more about聽, it鈥檚 vital to understand the risks human activity poses to this remarkable system of navigation.